1. Field of the Invention
This invention relates to an apparatus for positioning a substrate such as a semiconductor wafer or a photomask, and in particular to a positioning apparatus suitable for an exposure apparatus of which highly accurate positioning is required.
2. Description of the Prior Art
In the manufacture of semiconductor devices such as IC and LSI, there is known an exposure apparatus in which the image of a circuit pattern depicted on a negative called a reticle is reduced and projected onto the photosensitive layer of a semiconductor wafer by a projection lens. The size of the circuit pattern is of the order of 20 mm square at greatest and the diameter of the wafer is of the order of 100 mm-150 mm and therefore, in this apparatus, the transfer of the circuit pattern to the whole surface of the wafer is effected by the so-called step-and-repeat method in which the wafer is placed on a two-dimensionally movable stage and the wafer is caused to step relative to the projected image and then exposed to light.
Usually, to manufacture a semiconductor device, the projected image of a circuit pattern different from a previously transferred circuit pattern is superposedly exposed to light with respect to a wafer. Therefore, it is necessary to correct the two-dimensional alignment and rotational deviation relative to the projected image of the wafer before the superposed exposure is effected. The exposure apparatus of this type has incorporated therein a so-called precise alignment apparatus which detects an alignment mark provided on a wafer by a photoelectric microscope and effects the alignment between the wafer and the projected image. In order to shorten the time required for the searching of the alignment mark by this alignment apparatus, the wafer is roughly positioned by the use of a flat provided in a circumferential portion thereof before it is placed on a stage. This rough positioning is accomplished by a prealignment apparatus. This prealignment apparatus chiefly serves not only to accomplish two-dimensional rough positioning of the wafer, but also to orient the position of the flat of the conveyed wafer always constant relative to the direction of two-dimensional movement of the stage. When the orientation is accomplished, the wafer is conveyed from the prealignment apparatus to the stage so that the direction of the flat thereof does not change, and is held by a vacuum adsorption holder rotatably provided on the stage, whereby precise alignment including the correction of the rotational deviation is accomplished.